Either you or your friend is misspelling their address (more common than you might think, I get opt-in mailing-list mail for firstname.lastname@example.org, intended for email@example.com), or you've encountered a bug.
Google's own firstname.lastname@example.org mailing list has this as the "To:" field:
And at the bottom of the message:
To unsubscribe from this group, send an empty message.
I had to ctrl-u and check the "Delivered-To:" and "X-Forwarded-For:" headers before I could unsubscribe.
(I'd tried to unsubscribe previously but the subscribed email account forwards to my main account so replying with an empty message didn't work. This thread prompted me to dig a little deeper and finally get one less piece of email per day - thanks HN!)